Trying to make free shipping items, but not working

I have two products in the store that I want to always have free shipping. When I set a ‘free shipping’ class to these items and charge $0, it works fine. But as soon as they add another item with a different shipping class, it gives the option at check out for free shipping OR paid shipping (which no one will ever pick). Is there a way to make it so if a non-free shipping item is added, it will remove the free shipping option?